Integrated circuits typically comprise a number of layers formed on a silicon substrate. As integrated circuits become smaller, and the thickness of layers comprising the integrated circuits is reduced, the behavior of devices formed from these layers often depends on the thickness of a specific layer. For example, a transistor formed on a silicon substrate may have different characteristics depending on the thickness of the gate of the transistor. It may therefore be useful to determine a thickness of a layer in a microelectronic device such as an integrated circuit.
The thickness of a layer in a microelectronic device such as an integrated circuit may be determined using one of several techniques. The microelectronic device typically includes a structure including several layers built up over a substrate. Ellipsometry, using an electron probe with wavelength dispersive spectrometer(s), angle-resolved x-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS) are techniques that may be used to determine a thickness of a specific layer in a structure.
Ellipsometry includes directing polarized light at the surface of a structure, and measuring a shift in polarization of light reflected off of the surface. Ellipsometry may be difficult to use with very thin layers (e.g., less than 1 nanometer (nm)), because of weak optical response. Since layers are becoming increasingly thin, the applications of ellipsometry are becoming more limited. Further, ellipsometry can only determine the thickness of one layer in ultra-thin multi layer film structures.
An electron probe with wavelength dispersive spectrometer(s) irradiates a layer with medium-energy electrons. The thickness of multiple layers can be inferred by the measurement of characteristic x-rays corresponding to different layers. However, film damage is a concern because of the irradiation. Further, interfacial silicon oxide layers underneath an oxide (e.g., a silicon dioxide layer underneath a hafnium oxide layer) are difficult to measure accurately because the technique cannot distinguish between the different chemical states of silicon.
Angle-resolved XPS uses photoelectron spectroscopy to determine a thickness of a layer. Photoelectron spectroscopy bombards a sample with photons having a specific wavelength (here, x-ray photons), which excites the atoms of the sample to generate a photoelectron having a characteristic energy for the sample. The technique depends on measuring photoelectrons at different emission angles from the sample surface, for example by tilting the sample with respect to an electron energy analyzer. For metrology applications, the technique is expected to be deficient in meeting high measurement throughput requirements due to lack in signal intensity, which either results in poor measurement precision or long analysis time.
SIMS uses a focused ion beam directed toward the surface of a sample. The bombardment by low or medium energy ions leads to the ejection of both neutral and charged species from the surface of the sample. The ejected charged species are measured using a mass spectrometer by monitoring the signal intensity of one or more suitable ion species as a function of time. Assuming a constant material removal rate for a given material and primary ion current, the analysis time required to observe a defined change in signal intensity of a suitable ion species is converted into a depth scale, which is used to determine layer thickness. However, SIMS is a destructive process, as the species ejected and analyzed are a portion of the layer being measured.